Recently, various technologies have emerged that allow users to experience a blend of reality and virtual worlds. For example, head-mounted display (HMD) devices may include high definition cameras, depth cameras, range finders, gyroscopes, accelerometers, and other technologies that allow the HMD device to map the real world and to display a blend of reality and virtual objects on the HMD device. These HMD devices may include stereoscopic displays that use stereopsis to achieve 3D visuals by displaying separate images with appropriately translated positions to each eye of the user. When using stereopsis techniques, the perceived depth of the 3D visual will depend upon the inter-pupillary distance (IPD) of the user. Rather than calculating an accurate IPD for the user, stereoscopic HMD devices will often be calibrated to an average IPD for a population of users. Differences between a user's actual IPD and the average IPD may result in some changes in how the visuals are perceived by the user, as discussed below; however, those changes may be imperceptible when viewing an immersive virtual reality experience, as opposed to augmented reality experience, because the user has no frame of reference in the real world against which the displayed visuals can be compared. However, even in a virtual reality experience, an incorrect IPD may cause user discomfort. Additionally, if the IPD error is too large, the user may be unable to verge the left and right images displayed to the user's left and right eyes, causing the user to see double, and generally degrading the user's experience of the virtual reality.
The approach of using an average IPD for a user population has several drawbacks in HMD devices for displaying augmented reality experiences, which comprise at least partially see-through displays that allow superposition of displayed images over a real-world background environment. As the user is seeing the real-world, and not just a virtual representation of the real-world on a display, the real-world background environment provides a ground truth reference frame for the user. A deviation between the user's actual IPD and the IPD used by the HMD device will affect only the user's perception of the virtual objects displayed on the HMD device, while the ground truth real-world background environment will remain unchanged. Consequently, this imprecision in IPD can cause the virtual objects to either appear to be an incorrect size or appear to be in an incorrect location compared to the real-world background environment, degrading the user experience.